In my current D&D 4E game, I have a fairly large number of players with differing preferences for play style. As they have progressed through levels, differences between the characters with regards to their combat capability (particularly between those who are into "character optimization") have become drastically more pronounced making it difficult to create balanced encounters and to make everyone feel like they're contributing during combat situations (with some characters capable of dealing enough damage to kill a character of their same level). Given the amount of time required to handle combat, I'd like to ensure that combat situations are fulfilling for the entire group as much as possible.

What techniques exist (particularly as proposed by WotC or other industry sources) for handling the combat issues caused by this disparity?

You already know how to make combats tactically interesting. To make combats interesting to story gamers, add an interpersonal dimension. What are the issues that are important to your story gamer players (and their characters)? Push those issues.

Example:

The party encounters a horde of goblins in a dungeon. You have an encounter set up that will capture the interest of the power gamers. Now you need to make it interesting to the story gamers, like "Bob," the player of the paladin.

You add a dozen prisoners (all minion NPCs) tied up and spread out over the map. The goblins have them held hostage. A goblin can choose to perform a coup de grace on a prisoner instead of attack the PCs. A prisoner will move by themselves as long as he or she is adjacent to a PC (for protection), but provokes opportunity attacks as usual.

So now the paladin has to figure out how to save the prisoners. It's not just a combat anymore. It's a choice between helping his friends fight and helping the prisoners get away. It works because the paladin's "defender" role is thwarted by all the crazy movement required to move the prisoners.

Reward ALL the players for each rescue, with role-playing and dialog and effusive thank-yous from the grateful hostages. Reward them all with XP, too. This can be a small, flat XP award per hostage or you can treat the encounter as a minor quest.

Push Consequences

Story gamers love consequences. If your games are a series of barely connected combat encounters, the story gamers will grow bored.

Consequences glue player choices together. What you decided yesterday influences what happens today and tomorrow, for good or for bad. Story gamers want that cause-and-effect in their game. That connectedness is story.

You don't have to spring consequences on a player by surprise. It's often better to let them know exactly what will happen if they decide X or Y, and let them choose. Just make all the paths meaningful and interesting. Make the choice hard.

Example:

The paladin and his party have cleaned up the goblins and freed the hostages, but the goblin leader escaped. The surviving hostages tell the party, "You have to help our Baron! He was captured with us, and taken away to be tortured a little bit ago!"

Does the party chase the goblin leader or go rescue the Baron? Make it obvious that the paths lie in different directions. Make it obvious that the Baron will be tortured, possibly killed, if the party does not rescue him immediately. Make it obvious that the goblin leader will escape if not pursued immediately -- perhaps to go back to raid the unprotected village as punishment for the attack on his tribe.

Then the paladin and the rest of the party have a tough decision to make. The story gamers will eat this up.

Here's my tip for making combat more appealing to story players. Make the combats meaningful. Don't use filler combats just because you feel like it's time for a fight. Fights shouldn't happen just because a level appropriate monster showed up. They should happen because the players have a unresolvable conflict with an NPC and somebody has to die. When the PCs pick a fight they should know the names of at least one of the enemies. To make combats matter for story gamers, the fights need to be personal.

My second tip for this is to make it explicit that you can't entertain both groups of players at the same time. Your game will have story time and mechanics time. You'll appreciate it if the players bear with you for the portion that doesn't interest them, because they'll have their share of game time later.

Ideally you can give players something to do during this time. I was the lone story gamer in a group of brawlers several years ago. The GM found that if he had roleplaying to do with me, he should make sure to pass out loot immediately before talking to me. Dividing up magic items gave the others something to do while I had one on one GM time. On the flip side, if you want to give your roleplayers something to do during combat, having them write letters and send messages comes to mind (although that will require a bit of set up if you're going to make that a regular part of your game).

Assuming that the story focused players are not just building mechanically weak characters but rather focus on skills and utility stuff, my suggestions would be:

Spice up combat with skill challenges. Many players focusing on their combat effectiveness somewhat neglect their skills because skills usually don't help you to kill stuff. Add some traps, riddles, timed scripted events to your combats which the combat optimized characters can't defeat/solve on their own. This would give even the unoptimized characters an important role in the encounter.

Make combats less important and focus on social/knowledge encounters and skill challenges. If combat only happens every other or every third session it takes the wind out of the optimizers' sails very quickly because they just can't show off their characters' abilities.

Just tell the optimizers to cut it. If they don't, build encounters appropriate for the weaker characters. These encounters will obviously be trivially easy for the optimized characters which in turn will bore their players. Because players usually don't like boring encounters they should get the message very quickly.

If all else fails, TPK them all and make them build characters on the same power level. ;-)

Let your storytellers describe their exploits in combat. Reward the good storytelling (which is not the same as most flamboyant description or largest set of words!) by telling your own good stories (and maybe not out loud) about why their opponent became distracted or tripped or lost a piece of armor for a circumstantial bonus that put them on par with the powergamers. Let them powergame their storytelling.

It would be difficult and the responsibility would fall on you to keep it balanced and not overuse it, but, since you know exactly how "behind" they are, you know what kind of bonus range you have to work with.

And if your powergamers get jealous and start to epically describe what their characters do in hopes of getting a GM story in return, is that a bad thing?

Uhm... I don't remember any WotC suggestion specific for this case. If a player follows the guidelines defined in character creation I think that - even if not optimized - a PC shouldn't be much far behind a heavy minmaxer's one.

But, if one of your players is playing a Fighter with a Strength score of 8 because his father wanted him to take the way of the sword even if a plague crippled his body when he was young then, OK, there is an issue to be addressed.

If this is the case, filling such a gap could be really tricky. Following the character creation rules (even while ignoring the guidelines) provides a PC that has shining points.

Did the fighter in the example take the Linguist feat? Leverage this with a combat encounter where words provides an easiest and more remunerative (more XP) way to solve the fight.

Is the choice of physical ability scores totally wrong for the storytellers' PC? Start an adventure in the Mindscape and make the whole party generate a new mental counterpart of their PCs' sheets where physical abilities are replaced by mental one (Strength with Charisma, Constitution with Wisdom, Dexterity with Intelligence). This could also reduce the effectiveness of the min/maxers. Make sure the adventure ends at some point, or make the campaign alternate between physical and mental worlds.

Is their power selection very poor? Do they never hit? Are they always being beaten up by monsters? Introduce some weird monster type that works on reversal (that if is hit, it is missed, if it hits, it misses and vice versa). Don't abuse of it. ;)

These are a couple of advices that specifically address your question (balancing combat encounters). I took some guesses on the reasons that cause your storyteller's PC to hang back. If I missed something tell us and we could be more specific.

However, if the goal is to make everyone enjoy the game, follow through the already posted advices that provide storytellers a frame they could shine within: skill challenges, purely role-played sessions, great stories tightly intertwined with characters' backgrounds.